Method of investment casting



Nov. 10, 1964 J. HOCKIN METHOD OF INVESTMENT CASTING Filed May 16. 1951 mmmmmm III/II United States Patent F 3,156,023 METHGD OF INVESTMENT CASTING John Hockin, 4365 S. Maple Ave., Brookfield, Ill. Filed May 16, 1961, Ser. No. 110,417 4 Claims. (Cl. 22-196) This invention relates to a new and improved method of investment casting having as its primary purpose the elimination of mold imperfections.

Investment casting, as it is now called, has been previously referred to variously as the lost wax process and also as precision casting. In essence, the investment casting process involves the use of disposable pattern material such as heat disposable wax, plastic patterns, frozen mercury or the like, which are surrounded with refractory material to form a monolithic mold. Patterns are removed from the molds by heat provided by ovens or other means and molten metal added to form the casting. The casting is then easily removed by destruction of the inexpensive mold.

The process of investment casting is beginning to be widely used for particular casting problems because it provides an effective way of forming castings to particularly close dimensional tolerances; from whence it derives its name; precision casting, and with smooth surface finishes. Further, and perhaps a more important advantage, is that investment casting offers a wide range of design flexibility previously thought impossible with standard casting techniques. For example, investment casting frequently permits the use of a one-piece casting which obviates multipiece, assembled or welded components.

The economy realizable from investment castings is particularly augmented by its ready adaptation to use with a plurality of patterns gated to a central sprue permitting the molding of multiple duplicate pieces simultaneously in a single mold.

One of the basic drawbacks to investment casting has been a relatively high and heretofore most unpredictable Waste factor. That is, the percentage of rejected castings to the castings which would meet basic acceptance standards has been in a range higher than that experienced in other casting processes. Quite often these defects were of a type which could not be cured simply by inexpensive means.

Therefore, it is the primary object of this invention to provide an improved method of investment casting which significantly reduces mold imperfections which result in casting rejects.

Other objects and purposes of this invention will be apparent from the more particular description and from the drawings in which:

FIG. 1 is an enlarged cross sectional view of a pattern shown covered by a wet first dip coat;

FIG. 2 is an enlarged cross sectional view of a pattern covered by a dried first dip coat and a wet second dip coat;

FIG. 3 is an enlarged cross sectional view of a pattern covered by a thin film and a wet first dip coat; and

FIG. 4 is an enlarged cross'sectional view of a pattern covered with a thin film, a dried first dip coat and a wet second dip coat.

More particularly, investment casting has previously utilized a flask surrounding a tree of patterns gated to a central sprue, an investment mold slurry being filled into the flask surrounding the pattern and being permittedto set and .dry. Thereafter the patterns areremoved from the mold and the hot mold filled with metal. This procedure leads to so many mold imperfections due to the high viscosity of the investment mold slurry and its failure to conform precisely to the pattern that it has been for the most part. discarded in favor ofanother process, although it is still used to some extent. '7 1 3,156,023 Patented Nov. 10, 1964 The most widely used investment casting technique at the present time involves the utilization of one or more dip coats of relatively low viscosity applied in sequence directly to the pattern, each being permitted to dry before the next is applied. Then the pattern with the dried dip coats is placed in a flask and then surrounded with the more highly viscous investment mold slurry which is then dried to provide structural support to withstand the force of the molten metal it will receive.

While to a certain extent this decreased mold imperfection resulting from the direct use of the investment mold slurry, mold imperfections still occurred resulting in a high proportionality of unuseable castings. It has been and still is the teaching of those skilled in this art that to further reduce casting imperfections the adhesion between the first dip coat and the pattern must be maximized. T 0 this end the prior art went so far as to teach the roughening of the surfaces of the pattern to augment the adhesion between the pattern and the first dip coat. In fact, this statement is typical of the present status of the art:

The two most frequent causes of trouble are (l) mold release compound not properly cleaned off so that the first coat failed to wet the surface properly and therefore failed to adhere, and (2) lack of temperature control between the step of precoating and the time of applying the back-up of investment 1 However, the attempts to increase the adhesion between the pattern and the first dip coat did not result in reduction of casting imperfections.

Another isolated and previously unrelated observation of which the prior art was aware was that when the dip coats were dried under conditions of higher humidity, there were fewer casting imperfections than when they were dried under conditions of lower humidity. However, the tedious, slow drying of the dip coat under conditions of relatively high humidity was most undesirable from the standpoint of rapid production schedules. Further, the cost of maintaining and controlling high humidity during drying further added to the expense of the process.

However, a more particular description of the precise details of investment molding techniques is needed before the present invention can be properly appreciated.

In the investment casting process the pattern to be used is of course critical to the quality and characteristics of the final casting. This pattern is usually formed by injecting a heat disposable wax or plastic into a die to make a pattern. These patterns are usually so made that they can be easily attached to a central sprue by means of a separate gate, or by means of a gate formed on the pattem to form a tree of patterns.

cate, or silica sol as a binder. The tree of patterns is then removed from the slurry, the excess slurry being permitted todrip from the pattern, and then a granulated refractory is sprinkled or otherwise deposited on the wet precoat to form a rough surface to which the refractory back-up slurry, or the second dip coat, will, adhere, Ob-

viously with this technique, the first dip coat conforms quite preciselyto the contour of the pattern to assure an accurate mold. The tree of patterns is then dried in roomtemperature air. After the first dip coat has dried the tree of patterns is again dipped into the slurry, granulated Nalco Chemical Co. Release 8-777, revised September 1959. Also see Foundry, January 1957, at page 112, to same In addition to heat dis- 1 posable Wax, thermoplastic resins, usually polystyrene,

refractory again sprinkled on the wet second coat to form a rough surface to which the refractory back-up slurry, forming the major part of the mold, will adhere. Then the second dip coat is permitted to dry in room temperature air and the tree of patterns placed within a flask and the refractory back-up slurry poured into the flask surrounding the pattern.

The flask may then be vibrated or may be subjected to a vacuum to remove entrapped air bubbles from the slurry.

The molds are then allowed to set in room temperature air, the chemical action of the mold material hardening the mold.

The pattern material is then removed from the hardened mold. The most common method employed for removing the pattern material involves the placement of the mold in a high temperature oven, fired at 1300 to 1900 F. to remove all pattern material and carbonaceous matter. The molds remain in the high temperature ovens for from four to eighteen hours, the exact time being determined by the size of the mold, type of investment and complexity of patterns. Immediately after the final burn-out operation, molten metal is poured into the molds to form the castings. This pouring of the molten metal into hot molds makes possible the production of castings with very thin sections and exceptionally fine, sharp detail.

Also becoming more frequently used is what is referred to as the investment shell casting process. In this process the tree of patterns is subjected to repeated dippings in a slurry of refractory flour, generally 325 mesh, and a special liquid refractory binder. Dry refractory grain is sieved or otherwise deposited on the freshly coated tree between each dipping. This is called stuccoing. The refractory grain ranges in size from 20 to 70 mesh, the fine material being used for the initial coat and progressively coarser grains for subsequent coats. In this procedure the tree of patterns is not placed within a flask and the investment mold slurry not used, the mold attaining its structural strength from the plurality of dip coats.

Each dip coat is set before subsequent coats are applied, the end shell being built up to a total thickness of from W to /2", depending upon the complexity and weight of the desired casting.

After the molds are thoroughly dried in air, the pattern and gating material are removed either by placing the mold in an oven or by subjecting it to a heated trichlorethylene vapor bath or a hot liquid bath or other such method. The shell is then placed in a high temperature furnace at approximately 1800 to 2000 F. to remove all traces of organic material. The mold is again poured immediately upon removal from the furnace as previously described.

It is also important to consider that the resultant investment casting mold made according to any of the previously described methods is sutficiently porous that the air within the mold can escape readily through the sides of the mold when molten metal is poured in to prevent casting irregularities. Further, the porous nature of the mold permits the use of a vacuum to expedite air removal during the pouring operation.

One of the most readily observable and frequently occurring casting defects has been that of mold chips being found within the casting. This of course is impossible to economically correct and consequently results in rejection of the casting.

Previously, as has been described, it was thought that this flaking or breaking of the mold in some cases was the result of a failure to attain proper adhesion between the pattern and the first dip coat. However, contrary to this prevailing thought, I have found that the formation of a soft thin film completely surrounding the surface of the pattern prior to the application of the dip coats results in a very substantial and significant decrease in casting irregularities.

More particularly, I have utilized two different film forming solutions, one for wax patterns and yet another for plastic. After the wax patterns are appropriately sprued and treed, I dip them into a solution comprising substantially five percent kerosene, eighty-five percent alcohol and ten percent acetone, which is added to make the alcohol and kerosene miscible. Any volatile fluid such as alcohol very rapidly evaporates leaving a very thin, uniform soft film com letely covering the surface of the pattern. I then proceed with the first dip coat and when the first dip coat is completely dried and where required, a second dip coat is deposited on the pattern covered with the film in the manner previously described.

The second film forming solution I have used with plastic patterns comprises water plus a simple wetting agent comprising approximately one to five percent of the solution. Simple detergents are quite effective for use as a wetting agent and the procedure followed is otherwise the same.

In both the foregoing examples, it is believed that the film forming solutions being of low surface tension tend to give a more uniform deposit of film over the surface of the pattern than could be expected from a solution having a high surface tension.

It is to be particularly noted that silicones are quite frequently used in the dies in which the patterns are formed as a mold release and the industry-wide practice at the present time is to remove this silicon completely before the application of dip coats in order to achieve the desired adhesion between the dip coat and the pattern. The teaching of this invention is contrary to the prior art in that silicones, or any other solution forming a uniform soft film over the surface of the pattern to reduce the adhesion between the dip coats and the pattern below the cohesive forces within the dip coats is desirable. This will become more clear by referring to the drawings.

In FIG. 1 the central pattern 10 made of either plastic or wax as aforedescribed, is shown after the application of the first dip coat 12. This is representative of the old procedure wherein the dip coat 12 was permitted to dry directly on the surface of the pattern 19 to achieve maximum adhesion.

In FIG. 2 the pattern shown in FIG. 1 with its dried first dip coat 12 is then covered with the second dip coat 14. The drying of the second dip coat 14 cools the first dip coat and the pattern 10 invested therewithin. However, the coefficient of expansion of the pattern 10 is quite in excess of the coefficient of expansion of the first dip coat 12 Which results in the pattern 10 shrinking much more than does the first dip coat 12. This is indicated by the dotted line 10a in FIG. 2. Here, where the adhesive forces between the pattern 10 and the first dip coat 12 exceed the internal cohesive forces within dip coat 12, it is seen from FIG. 2 at 16 that the dip coat 12 will partially adhere to the pattern during its shrinkage. Since the coefficients of expansion of the pattern 10 and the first dip coat 12 are critically different, the dip coat must partially break up as shown by the broken lines 16 to accommodate the adhesion of the dip coat to the shrinking pattern. These chips and breaks which are produced at this time of course produce mold irregularities which are imparted to the final casting to produce most frequently an unuseable casting. Further, these chips may break loose and actually fall into the molten metal poured into the casting after the pattern 10 has been removed to produce defects undetectable by mere visual inspection.

Thus, the teachings of the prior are that a high degree of adhesion must be maintained between the first dip coat and the pattern, while tending to produce a highly precise and accurate mold, actually contributed substantially to the production of mold irregularities.

It is the primary teaching of this invention that in order to avoid mold irregularities caused by the differences in coefficients of expansion between the pattern material and the first dip coat material, the process of forming the mold must include the deposition of a film on the pattern as shown in FIG. 3 at 18. This film must of necessity be extremely thin in order not to in any way aberrate, change or modify the pattern configuration. Further, as before mentioned, this film must significantly reduce or entirely eliminate the adhesion between the first dip coat and the pattern surface to less than the cohesive forces tending to hold the first dried dip coat together internally. Said in yet other words, the pattern must be free to shrink or contract independently of the shrinkage or contraction of the dried first dip coat.

Also, a criticality of the film is that when the said first dip coat is applied over the film covered pattern, the film must be sufiiciently insoluble, relatively, that the first dip coat will not dissolve it during the drying of that dip coat. In other words, the rate of solution of the film must be such that the drying of the first dip coat precedes the solution of the film by the first dip coat. It is essential that the film remain covering the entire surface of the pattern after the first dip coat is dried. Second and third or any desired number of dip coats may be thereafter applied and the casing procedure completed as before described.

From the foregoing and from FIG. 4, it will thus be seen that whether the drying of the mold is done under conditions of high humidity or low, and the shrinkage of the pattern occurs rapidly or slowly, it can do so independently of the first dip coat, hence preserving the integrity or structural strength of the first dip coat to provide highly uniform, significantly defect-free molds and obviate consequent casting rejects.

I claim:

1. In a method of investment casting the steps of forming a pattern, immersing the pattern in a film forming solution to form a soft thin insoluble film of uniform thickness covering the entire surface of the pattern, covering said pattern with a dip coat comprised of refractory and a binder, drying said dip coat while maintaining said film, covering said pattern with a second dip coat and drying said second dip coat, said film serving to minimize adhesion between the first dip coat and the pattern.

2. In a method of making an investment casting the steps of forming a pattern, depositing and maintaining a soft thin insoluble film of uniform thickness on said pattern, covering said film covered pattern with a first dip coat comprised of refractory and a binder, drying said first dip coat, covering said pattern with a second dip coat, and drying said second dip coat, said film serving to minimize adhesion between the first dip coat and the pattern.

3. In a method of investment casting the steps of forming a wax pattern, immersing said pattern in a solution consisting substantially of five percent kerosene, eighty five percent alcohol and ten percent acetone, removing said pattern from said solution, drying said solution on said pattern to form a uniform thickness of soft thin insoluble film over the entire surface of the pattern, depositing a first dip coat comprised of refractory and a binder over the entire surface of said film covering said pattern, and depositing a second dip coat comprised of refractory and a binder over the entire outer surface of said first dip coat, said film serving to minimize adhesion between the first dip coat and the pattern.

4. In a method of investment casting the steps of forming a plastic pattern, immersing said pattern in a solution consisting of water and substantially from one to five percent wetting agent, removing said pattern from said wetting agent, drying said wetting agent on said pattern to form a substantially uniform thickness of soft thin insoluble film over the complete surface thereof, covering said film covered pattern with a first dip coat comprised of refractory and a binder, drying said dip coat, covering said pattern with a second dip coat comprised of refractory and a binder and drying said second dip coat, said film serving to minimize adhesion between the first dip coat and the pattern.

References Cited by the Examiner UNITED STATES PATENTS 2,480,896 9/49 Bean l17-5.1 XR 2,752,257 6/56 Bradley et a1. 22193 2,908,952 10/59 Benham 117-51 XR 2,914,823 12/59 Bean 117-5.1 3,051,669 8/62 Emblem et al 22193 FOREIGN PATENTS 481,025 2/52 Canada.

MICHAEL V. BRINDISI, Primary Examiner.

RICHARD D. NEVIUS, MARCUS U. LYONS,

Examiners. 

3. IN A METHOD OF INVESTMENT CATING THE STEPS OF FORMING A WAX PATTERN, IMMERSING SAID PATTERN IN A SOLUTION CONSISTING SUBSTANTIALLY OF FIVE PERCENT KEROSENE, EIGHTY FIVE PERCENT ALCOHOL AND TEN PERCENT ACETONE, REMOVING SAID PATTERN FROM SAID SOLUTION, DRYING SAID SOLUTION ON SAID PATTERN TO FORM A UNIFORM THICKNESS OF SOFT THIN INSOLUBLE FILM OVER THE ENTIRE SURFACE OF THE PATTERN, DEPOSITING A FIRST DIP COAT COMPRISED OF REFRACTORY AND BINDER OVER THE ENTIRE SURFACE OF SAID FILM COVERING SAID PATTERN, AND DEPOSITING A SECOND DIP COAT COMPRISED OF REFRACTORY AND A BINDER OVER THE ENTIRE OUTER SURFACE OF SAID FIRST DIP COAT, SAID FILM SERVING TO MINIMIZE ADHESION BETWEEN THE FIRST DIP COAT AND THE PATTERN. 